On Mon, Nov 21, 2016 at 09:38:36AM -0800, Denny Page wrote:
> Given host A on a 1Gb link and host B on a 100Mb link:
>   A -> Switch transmission time 754ns
>   Switch -> B transmission time 7540ns
>   Total time 8294ns

As I understand it, modern switches don't wait until they have whole
packet before forwading it to another port. They just need to wait for
the part that contains the destination MAC address. So the total time
should be closer to 7540 ns.

>   B -> Switch transmission time 7540ns
>   Switch -> transmission time 754ns
>   Total time 8294ns

Yes, in this case the switch has to wait as it can't connect a 100mbit
input directly to a 1gbit output.

> The propagation delay is symmetrical.
> Unless there are a backlog of packets on one of the involved ports, the 
> switch has zero impact on accuracy.

I think your own experiments showed there is a 350 ns offset :). That
would be 700 extra nanoseconds in the B->A direction.

Miroslav Lichvar

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